1. Field of the Invention
The present invention relates to light-emitting elements using electroluminescence. In addition, the present invention relates to light-emitting devices and electronic devices including the light-emitting elements.
2. Description of the Related Art
In recent years, extensive research and development on the light-emitting elements using electroluminescence have been carried out. In a basic structure of the light-emitting elements, a light-emitting substance is interposed between a pair of electrodes. When voltage is applied to the elements, the light-emitting substance can emit light.
Since the above light-emitting element is of a self-light-emitting type, it has advantages that the visibility of its pixels is higher than that of a liquid crystal display, that a backlight is not required, and the like. Because of the advantages, the light-emitting element is known to be suitable for a flat panel display element. Another major advantage of the light-emitting element is that it can be manufactured to be thin and lightweight. In addition, the light-emitting element has a feature that its response speed is extremely high.
The light-emitting element can be formed into a film form. When the element is formed to have a large area, plane emission can be easily obtained. This feature is hard to obtain from point sources typified by an incandescent lamp and an LED, or linear sources typified by a fluorescent light. Thus, the light-emitting element is of value as a plane light source that can be applied to lighting and the like.
Light-emitting elements using electroluminescence are broadly classified depending on whether the light-emitting substance is an organic compound or an inorganic compound.
In a case where an organic compound is used as the light-emitting substance, by the application of voltage to the light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing the light-emitting organic compound, and current flows. When these carriers (electrons and holes) are recombined, the light-emitting organic compound is excited. When the excited state returns to a ground state, light emission is obtained. Owing to the above mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element.
Note that types of excited states of an organic compound include a singlet excited state and a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state, phosphorescence.
Problems of such a light-emitting element mainly originate from unsatisfactory performance of materials. In order to enhance the element characteristic, structures of the light-emitting element and materials used therein have been developed.
For example, according to Non-Patent Document 1, a method called Triplet Harvesting was used to form a highly efficient light-emitting element.